1. Field of the Invention
This invention relates to a method and apparatus for analyzing a gas, and, in particular, to a photometric method and apparatus for analyzing a sample gas for a particular constituent which is present in very low concentrations.
2. Discussion of the Prior Art
Continuous gas analysis techniques, particularly photometric techniques, are sometimes limited to the detection of those components in a gas which are present in concentrations greater than about 100 parts per million (ppm). With the growing concern over atmospheric pollants, particularly nitrogen oxides (NO.sub.x) and sulphur dioxide (SO.sub.2), however, there is growing desire to be able to determine when small quantities of certain gaseous materials are being introduced into the atmosphere. Hence, there is a need for highly sensitive instruments and techniques capable of detecting low concentrations of certain constituents in a sample gas.
The present invention is applicable, broadly, to instruments and techniques utilizing a variety of detecting methods. It is also applicable broadly to the analysis of any gas constituent for which detection techniques are available. For convenience, however, the following disclosure will be limited to a discussion of a method and apparatus for photometrically determining the NO.sub.x content of a gas. A discussion of such a method and apparatus will suffice to disclose the invention, and it is well within the skill of those skilled in the art to modify the disclosure and apply the invention to methods and apparatus for detecting different gas constituents using different detection devices.
The NO.sub.2 content of a gas has previously been photometrically determined using visible radiation falling within the broad NO.sub.2 absorption band centered at about 390 NM. NO, however, does not absorb strongly in this wavelength band, and is hence undetectable by this procedure. A technique for analyzing a sample gas for NO is described in U.S. Pat. No. 3,718,429 which issued on Feb. 27, 1973 to James A. Williamson, Jr. and is assigned to the assignee of the present invention. The method described in this patent proceeds by introducing the sample gas into a sample cell, introducing an oxygen-containing gas under pressure into the sample cell to bring about conversion of the NO to NO.sub.2, and then detecting NO by observing the increase in absorbance brought about by the conversion of NO to NO.sub.2. The use of oxygen to convert NO to NO.sub.2 was disclosed by S. W. Nicksic et al in an article in Analytical Chemistry, 34, 987 (1962) in which a sample gas was collected in a syringe, mixed with an equal volume of oxygen, then compressed for the prescribed time (usually about 15 minutes) or allowed to stand before being subjected to photometric analysis.
The techniques described above were used to effectuate conversion of NO to NO.sub.2 and did not increase the absolute sensitivity of the method and apparatus for NO.sub.x. It is the object of this invention to provide a method and apparatus to extend the sensitivity of analytical determinations, particularly photometric determinations, so that analysis, presently limited to a minimum full scale range of 100 parts per million (ppm), can be extended to full scale ranges of the order of 20 ppm.